A “biosensor” has been defined as an analytical device incorporating a biological or biologically-derived sensing element either integrated within or intimately associated with a physicochemical transducer. Biosensors are generally designed to produce either discrete or continuous digital electronic signals that are proportional to a single analyte or a related group of analytes, although the provision of analogue signals should not be excluded.
There are many areas of application for biosensors including for example environmental sensing, chemical production, and food and drink production and preparation. One area of application that has attracted a great deal of interest however is that of medical diagnostics, monitoring, and treatment. The following discussion addresses primarily these medical applications, although it will be appreciated that the problems and solutions considered may also have non-medical applications.
A typical example of a medical monitoring biosensor is the glucose biosensor that is designed to produce an electrical signal indicative of the level of glucose present in a user's (i.e. the patient's) system. Today's glucose biosensors tend to be based around the concept of immobilising an enzyme or other reagent on the surface of an electrode to provide what is essentially a pH detector. When the reagent is exposed to a sample obtained from the patient, e.g. a drop of blood, the electrical output of the device indicates the pH value of the sample and hence indirectly the level of glucose. Commercially available glucose biosensors tend to be handheld type devices which accept a disposable test strip or element.
A user may be expected, e.g. in the case of a diabetes sufferer, to test his or her glucose level several times a day in order to provide a sufficient degree of feedback to allow intervention if the detected level deviates significantly from the “normal” level. Biosensors of this type have their limitations. In particular, due to the need for users to prick their skin to obtain a blood sample, and to then perform a short but still inconvenient test procedure using the biosensor, users may not perform the test as often as required. Skin pricking is also painful and, over the long term, can result in serious skin damage. These problems apply equally to other types of biosensors which measure analytes present in blood and thus require the provision of blood samples; for example the measurement of oxygen, lactate, nitric acid, creatine, dopamine, serotonin, noradrenaline. The measurement of these analytes is useful in the understanding and monitoring of diseases as diverse as heart disease, rheumatoid arthritis and Parkinsons disease.
Substantially non-invasive biosensors have been proposed. These might be wearable on the skin, making contact with interstitial fluid drawn through the upper layers of the skin by tiny micro-needles to provide continuous monitoring. However, the nature of such biosensors, being exposed to dirt and water and being subject to aggressive physical contact, e.g. via a user's clothes, is likely to require that the sensors be disposable, being used for only a relatively short period of time. Power consumption and battery capacity may also make disposability the preferred option. Two-part biosensor systems have been proposed that use a radio frequency wireless link to transfer data between a wearable biosensor and a central controller. Such a controller might be carried in a user's pocket or worn on his or her belt. The controller displays measured results on a display screen and may log historical data.
A typical biosensor system requires calibration of a biosensor prior to use in order to compensate for device variations and to ensure the accuracy of results. This might require, for example, measuring a parameter of a known fluid sample. In the case of a glucose biosensor, a user might be provided with a vial containing liquid with a known glucose concentration, the user breaking a seal and pouring the liquid onto the active sensor surface to calibrate the system. This approach is both time consuming and inconvenient for the user, and requires that the user be provided with a fresh vial for every biosensor.
U.S. Pat. No. 6,441,747 describes a wireless programmable system for medical monitoring that includes a base unit designed to communicate with a plurality of worn biosensor transceivers.
US2004/0096959 describes a glucose sensor in the form of a skin patch having a microneedle which penetrates the skin to draw out interstitial fluid. Glucose measurements are sent from the patch to a remote display unit, over a wireless link.
Other documents relevant to this field are:    IEEE Trans Biomed Eng, vol 35, no 7, July 1988, p 526-532;    Diabetes Technol Ther, vol 1, no 3, 1999, p 261-6;    Med Eng Phys, vol 18, no 8, 1996 December, p 632-40;    US20010041831; and    WO2000067633.